1. Field of the Invention
The present invention is directed to a method of modifying or reshaping the acoustical signature created by the explosive, underwater release of a highly pressurized gas. More specifically, the present invention is directed at a method for modifying the asymmetric and cuspate nature of the acoustical signature derivative from the generation of a primary pulse so as to allow for the beneficial superimposition of a second wavelet of opposite phase.
2. Background
When a charge of highly pressurized gas is explosively released underwater, a powerful and useful primary pressure pulse is created, said pulse being followed by a pulsating tail with several secondary pressure pulses. These secondary pulses constitute a pulsating tail which is generally cuspate in shape. The asymmetrical nature of this signature tail generally complicates the reduction of its amplitude to an acceptable level for purposes of seismic exploration.
Methods have previously been developed to address the reduction or suppression of this asymmetrical secondary pressure pulse. In U.S. Pat. No. 4,735,821, ("the '821 patent") techniques are set forth for substantially eliminating the emission of the pulsating tail. This reduction is accomplished by tuning the selective injection of a second charge of highly pressured gas within the bubble created by the release of the primary charge.
Alternatives to this method, however, are desirable in a variety of circumstances. For example, it may sometimes be desirable to shape the overall acoustic signature on a shot-by-shot basis. Further, it may be desirable to suppress the spectrum oscillation of the signature, phase modulate, or invert the signature on a shot-to-shot basis to achieve a more stable P.sub.2 /P.sub.0 ratio. Alteration of the method described in the '821 patent may also be desirable from the standpoint of beam steering the acoustic wave.
Incentives to vary from the teaching of the '821 patent also include the possibility of using a minimum number of identical acoustic generators in an array. Not insignificantly, it may also be desirable to deviate from the above described methodology for purposes of energy conservation.